Such gripping members are commonly used in vessel conveying installations, in particular for conveying thermoplastic preforms through an oven.
Such preforms are generally obtained by injection and have a tubular cylindrical body sealed at one of its axial ends, and which is prolonged at its other end by a neck, which is also tubular. The neck is generally injected in such a way as to already have its final form whereas the body of the preform is required to undergo a relatively great deformation to form the final vessel following a blowing operation.
The neck of the preform often has a bearing collar cooperating with a gripping clamp or with a guiding rail. The thinned edge at the open axial end of the neck is called the “rim” of the vessel. The axial distance between the end of the rim and the underside of the collar is generally a very precise distance because it is used for the sealing stopper of the vessel.
The operation involving blowing the body of the preform requires the latter to be raised to a temperature above the glass transition temperature of the material. The preform is subjected to a heat conditioning by having it circulate inside an oven. The oven comprises heating means which are, for example, formed by infrared lamps in which the preform is displaced by a conveying system.
The gripping members intended for conveying systems passing through an oven are not generally simple gripping clamps because the gripping member has to make it possible for the object to be driven in rotation about an axis.
At the end of travel, the mandrel has to be able to be withdrawn from the preform in order for the latter to be able to be transferred to a blowing station. Such an operation is known by the term “uncladding”.
A number of solutions are known for uncladding a preform. One of them consists in arranging an ejection plate above each mandrel. The mandrel is capable of being axially retracted in a central orifice of the plate. The rim of the preform then abuts against the plate. The plate then prevents the axial sliding of the preform, which causes the mandrel to be removed from the neck of the preform.
Such a large series bottle manufacturing installation is very costly. One and the same installation therefore has to be able to be used to manufacture bottles of varying dimensions. The diameter of the preform necks is notably likely to vary depending on the model of bottle to be manufactured.
To enable necks of different formats to be held, it is known practice to change the mandrels on the conveying devices.
Furthermore, it is necessary to also change the ejection plates because the diameter of their central orifice is not suited to all the neck dimensions. A central orifice that is a little too big relative to the diameter of the neck can in fact cause the preform to topple.
However, the ejection plate and mandrel changing operations are a tedious operation which requires the installation to be stopped for a lengthy interval.